It is known that after forming an air-fuel mixture around a spark plug and performing spark ignition, fuel is directly injected into a combustion chamber to cause compression autoignition combustion (for example, refer to Patent Document 1).
In addition, it is known that combustion by flame propagation is performed by subjecting an air-fuel premixture to spark ignition during a light-load operation and that diffusion combustion by compression autoignition is performed during a heavy-load operation for example, refer to Patent Document 2). Subsequently, switching between the combustion by flame propagation and the diffusion combustion is performed after forcibly igniting pilot injected fuel to burn an air-fuel mixture and subjecting main injected fuel to compression autoignition diffusion combustion.
Furthermore, it is known that in an operating range in which knocking is likely to occur, a first fuel injection is performed into a cylinder before a compression top dead center in order to burn injected fuel by spark ignition and a second fuel injection is performed after the first fuel injection is completed in order to burn the injected fuel by diffusion combustion (for example, refer to Patent Document 3).
Moreover, it is known that a spark plug is provided so that an ignition electrode unit is positioned inside or in a vicinity of a spray of fuel injected from a fuel injection valve, the fuel is ignited by the spark plug after fuel injection to create a flame, one or more sprays is newly subjected to diffusion combustion by the created flame, and after all injections in a combustion stroke is completed, ignition is newly performed at least one or more times (for example, refer to Patent Document 4).
In addition, it is known that in an internal combustion engine performing homogeneous charge compression ignition (HCCI), when there is a cylinder whose combustion state deviates to a misfiring side from a prescribed range, ignition assistance is performed by a spark plug (for example, refer to Patent Document 5). However, with an internal combustion engine performing homogeneous charge compression ignition, since the inside of the cylinder is uniformly set to an air-fuel ratio (lean air-fuel ratio) that is significantly higher than a stoichiometric air-fuel ratio, ignition using a spark plug is difficult.
Heat efficiency can be increased by performing diffusion combustion by compression autoignition after performing combustion by spark ignition. However, an operating range which satisfies diffusion combustion is narrow. In addition, when the internal combustion engine falls outside of this operating range, there is a possibility that the diffusion combustion may not occur and unburned fuel may be discharged. Furthermore, there is a possibility that the diffusion combustion may not occur due to variations in temperature and pressure inside a combustion chamber after performing the combustion by spark ignition. When diffusion combustion does not occur, since mixing of fuel and air proceeds, there is a possibly that a large amount of fuel may automatically ignite at once in a similar manner to homogeneous charge compression ignition (HCCI). As a result, an abrupt increase in pressure may occur during combustion. In addition, combustion noise may increase due to the increase in pressure.